The present invention relates to a frequency shift keying (FSK) demodulation circuit, more particularly, to an FSK demodulation circuit which performs phase detection of an FSK modulated signal and demodulates a base band digital signal.
FSK modulation is one of the digital frequency modulation (FM) methods. It allots f.sub.C +f.sub.D (=f.sub.1) for one of the two logics of a base band digital signal (f.sub.C being the carrier wave frequency and f.sub.D being the frequency deviation), allots f.sub.C -f.sub.D =f.sub.0 for the other logic, and makes the signal f.sub.1 or f.sub.0 correspond to the two logics for transmission of information. Known FSK signal modulation methods include the synchronous detection method, the FM discriminator detection method, etc. The FM discriminator detection method involves a large number of discrete parts in the intermediate frequency band, which interferes with reduction of circuit size. Therefore, the present invention bases itself on the synchronous detection method, which is convenient for circuit size reduction.
When considering a synchronous detection method, if the base band signal speed is f.sub.B and the modulation index I.sub.DX (I.sub.DX =2f.sub.D /f.sub.B) is small, a carrier recovery operation is required. However, when the modulation index I.sub.DX is comparatively large (I.sub.DX =2f.sub.D /f.sub.B .gtoreq.5 or so, that is, when there are five or more sampling pulses in the data symbol), it is known that there is no longer a need for the above-mentioned carrier recovery operation in the demodulation operation. The present invention alludes to a simple, compact FSK demodulation circuit under these conditions. Giving an example, the FSK demodulation circuit is suited for a personal radio paging service, usually called a pocket bell or pocket pager.
One of the basic circuit portions comprising an FSK demodulation circuit is the data reproducing part, which reproduces the code of the original signal. The means for realization of the data reproducing part include, as typical conventional methods:
(1) the method of using a phase shifter and multiplier
(2) the method of using a D-type flip-flop
However, as will be mentioned in detail later, of the above conventional methods, the method (1) has a problem of a difficulty in realizing the phase shifter for delaying the phase of the base band signal. That is, the frequency of the base band signal is equal to the modulation frequency and usually about several kHz, so production of a phase shifter which shifts by exactly .pi./2 at such a low frequency is difficult.
Further, the above method (2) involves a simple circuit structure, so is readily realizable, but the circuit does not have a filter, etc., so suffers from the problem of a susceptibility to errors in the reproduced output due to noise.